The present invention relates to a Raman-amplified optical transmission system and, more particularly, to such a system with reduced four-waving mixing (FWM) by judicious choice of pump wavelengths with respect to signal wavelength(s).
It has previously been shown that performance of optical transmission systems can be dramatically improved with the use of distributed Raman amplification. Raman amplification, in general, is a nonlinear optical process in which an intense pump wave is injected into an optical fiber that is carrying one or more optical information signals. If the pump wave is of a frequency approximately 13 THz lower than the signal waves (i.e., if the pump wavelength is roughly 100 nm shorter than the signal wavelength in the vicinity of 1500 nm), the pump will amplify the signal(s) via stimulated Raman scattering. If the amplification is made to occur in the transmission fiber itself, the amplifier is known as a distributed amplifier. Alternatively, a lumped or discrete amplifier can be constructed with a local length of Raman gain fiber. By amplifying the signals within the transmission span, the signals can be amplified before their signal-to-noise ratio degrades to an unacceptable level.
An advantage of Raman amplification is that a set of pump wavelengths may be combined to broaden and flatten the gain spectrum associated with the signal wavelengths. In a conventional optical transmission system, the signal wavelengths may fall within the range of 1520 to 1620 nm. Presuming that multiple pump wavelengths are used and that it is desirable to use wavelengths approximately 100 nm less than the signal wavelength, a set of pump wavelengths within the range of 1420 and 1520 nm would be associated with this signal wavelength regime. It has been discovered, however, that four-wave mixing of the pump wavelengths will occur, that is, when the two monochromatic (or quasi-monochromatic) pump wavelengths mix, and result in generation of light at the sum and difference frequencies associated with these two waves. Without further consideration, these sum and difference frequencies may reside in the same band as the input signals, an undesirable result.
In addition, there are examples in the art where multiple Raman pump wavelengths have been used to broaden the composite Raman gain spectrum. An article entitled xe2x80x9cBroadband lossless DCF using Raman amplification pumped by multichannel WDM laser diodesxe2x80x9d by Emori et al. appearing in Elec. Lett., Vol. 34, 1998 at p. 2145 describes an arrangement where four pumps at wavelengths 1435, 1450, 1465 and 1480 nm are utilized. They demonstrate that a substantially broader and flatter gain spectrum can be generated with this pump configuration, but no mention of the problems associated with the generation of four-wave mixing products is made. For this particular configuration, a four-wave mixing component would be generated at 1529 nm due to the mixing of the 1435 and 1480 nm waves. It is entirely possible that the C-band signals could extend to 1529 nm. In fact, the authors state that the dispersion-compensating fiber used as the Raman gain fiber can compensate the dispersion of conventional transmission fiber from 1520-1600 nm. Additional work by Emori et al. extended the same approach to include twelve pump wavelengths extending from 1405 to 1495 nm. This source would potentially generate deleterious FWM components throughout the C- and L-bands. Again, no mention of four-wave mixing among the pump wavelengths is made by the authors.
Thus, a need remains in the prior art for a Raman amplified system using multiple pump signals that avoids the above-described four-wave mixing problem.
The need remaining in the prior art is addressed by the present invention, which relates to a Raman-amplified optical transmission system and, more particularly, to such a system with reduced four-waving mixing (FWM) by judicious choice of pump wavelengths with respect to signal wavelength(s).
In accordance with the present invention, the pump wavelengths are chosen such that the sidebands associated with four-wave mixing products occur at wavelengths shorter than the predefined signal wavelengths or, alternatively, fall between predefined signal wavelength ranges.
In a preferred embodiment of the present invention, the signal wavelengths are defined as falling within two predetermined signal bands: a C-band (1530-1562 nm) and an L-band (1574-1604 nm). A guard band of approximately 1562-1574 nm is thus created between these two signal bands. The pump waves in accordance with the present invention, therefore, are chosen such that the four-wave mixing products either remain below 1530 nm or fall in the guard band of 1562-1574 nm. Pump wavelengths of 1447 nm and 1485 nm may be used to provide relatively flat gain while maintaining all four-wave mixing products below the C-band lower limit of 1530 nm. In another embodiment, pump wavelengths of 1442 and 1502 nm may be used, where the four-wave mixing products will reside within the guard band (1562-1574 nm).
In any embodiment of the present invention, the pump wavelengths are chosen to provide a relatively wide and flat composite gain spectrum, defined by a width at least 50% greater than that generated by a monochromatic pump, where the composite gain width is measured at the gain equal to one half the peak gain in decibels.
Other and further embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.